The invention relates to a colour display device comprising a cathode ray tube having a display screen, a means for generating at least one electron beam and a deflection unit for generating deflection fields for deflecting electron beam(s) across the display screen in two perpendicular directions, and having magnetic field-generating means at or near a display screen-facing end of the deflection unit for generating an electromagnetic field to reduce raster distortions.
The invention also relates to a deflection unit for a cathode ray tube.
A colour display device and a deflection unit as described above are known from U.S. Pat. No. 4,746,837.
The known display device comprises a number of pole shoes arranged around the defection unit and at the side of the deflection unit facing the display screen. A pin-cushion shaped distortion of the deflection field is formed between the pole shoes. Said pin-cushion distortion necessitates a raster correction.
Although the known devices and similar devices in which magnetic correction fields are provided substantially reduce raster errors especially in the corners of the display screen, remaining raster errors are still noticeable.
It is an object of the invention to provide a display device and/or a deflection unit for a display device in which improved raster corrections are obtainable.
To this end, in accordance with an aspect of the invention, the display device is characterized in that the magnetic field-generating means comprise correction electromagnets, said correction electromagnets extending along an arc portion between angles xcex11 and xcex12, said angles obeying the following rules:
|cos(3xcex11)xe2x88x92cos(3xcex12)|xe2x89xa71.33
and
xe2x80x83|cos(5xcex11)xe2x88x92cos(5xcex12)|xe2x89xa60.5,
xcex11 and xcex12 being taken with respect to the line (horizontal) deflection plane, and the display device comprising means for driving the electromagnets, the electromagnets and the means being arranged to generate a correction field that is substantially mirror-symmetrical with respect to the line (horizontal) deflection plane, and substantially anti-mirror-symmetrical with respect to the field (vertical) deflection plane.
Correction magnets which extend through angles obeying the above rules generate a relatively strong six-pole field (to compensate raster distortions), i.e. at least ⅔ of the maximum, while generating a relatively small ten-pole field, i.e. less than 25% of the maximum ten-pole field. Such ten-pole fields may in themselves be a cause of distortions.
Each electromagnet preferably comprises a coil wound around a core, the coils being driven in operation by a current at the same ground frequency as the line deflection coils.
Several preferred sub-ranges exist within the indicated range for xcex11 and xcex12.
The first of such a preferred sub-range is given by the condition:
|cos(7xcex11)xe2x88x92cos(7xcex12)|xe2x89xa60.67
Within this range, the correction coils generate a relatively small 14-pole field (less than ⅓ of the maximum value). A somewhat larger range (up to ⅓ of the maximum) is possible since, in general, 14-pole fields are less strong than 10-pole fields.
A further preferred sub-range is given by
|cos(9xcex11)xe2x88x92cos(9xcex12)|xe2x89xa70.67
Within this sub-range, 18-pole fields are less than ⅓ of the maximum.